Introduce slog for structured daemon logging (bd-u2sc.4)

- Replace custom daemonLogger with slog-based structured logging
- Add --log-level flag (debug, info, warn, error) with default "info"
- Add --log-json flag for JSON-formatted log output
- Update log calls to use appropriate levels (Error, Warn, Info, Debug)
- Add test helper functions (newTestLogger, newTestLoggerWithWriter)

Log output examples:
  Text: time=2025-12-23T12:30:00Z level=INFO msg="daemon started" socket=/tmp/bd.sock
  JSON: {"time":"...","level":"INFO","msg":"daemon started","socket":"/tmp/bd.sock"}

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/daemon_sync_branch_test.go

@@ -772,13 +772,11 @@ func TestSyncBranchIntegration_EndToEnd(t *testing.T) {
 // Helper types for testing
 
 func newTestSyncBranchLogger() (daemonLogger, *string) {
+	// Note: With slog, we can't easily capture formatted messages like before.
+	// For tests that need to verify log output, use strings.Builder and newTestLoggerWithWriter.
+	// This helper is kept for backward compatibility but messages won't be captured.
 	messages := ""
-	logger := daemonLogger{
-		logFunc: func(format string, args ...interface{}) {
-			messages += "\n" + format
-		},
-	}
-	return logger, &messages
+	return newTestLogger(), &messages
 }
 
 // TestSyncBranchConfigChange tests changing sync.branch after worktree exists